PCs are not selling as they used to, though. iPads and other tablets have become attractive substitutes for many PC functions, like web browsing, watching videos and reading emails. Last year, global shipments of PCs fell 10% from the year before, while tablet sales grew 68%, according to the research firm Gartner.

During the holiday quarter, Microsoft said, Office revenue from businesses rose 10% while consumer revenue fell 24%, partly because it now sells Office as a subscription, which affects its accounting. While Microsoft no longer reveals how much total revenue it gets from the product, the company's business division reported nearly $25 billion for the fiscal year that ended June 30; 90% of that division's revenue was from Office.

Rick Sherlund, an analyst with Nomura Securities, says he believes that during the next year or so Microsoft could add about $1 billion of additional revenue from Office for the iPad and other devices that do not run Windows.

While it would take more than that to make a big difference for a company of Microsoft's size, he said investors believed that Microsoft was showing a greater willingness to create products for other companies' devices.

Adam Tratt, chief executive of Haiku Deck, a presentation app for the iPad that competes with PowerPoint, sees Microsoft's foray into iPad apps as largely a defensive move, rather than a chance to generate incremental revenue.

"The opportunity is to not lose a $25 billion market," said Tratt, who was an Office product manager in the 1990s.

Microsoft's previous chief executive, Steve Ballmer, said late last year that the company was working on Office for the iPad.

Microsoft has created software for other devices for years. It made Word for the Macintosh in the mid-1980s, and more recently released iPhone apps for its Bing search engine and other services. It offered an iPad version of OneNote, a note-taking application in the Office suite, more than two years ago.

But the iPad and other tablets represent a bigger potential threat to Windows than the Mac, which made bringing the most prominent Office applications to the devices a tough call.

For more than two years, Microsoft has had prototypes of Office apps for the iPad working inside the company, according to several people briefed on the products, who asked to remain anonymous discussing the internal development process.

The company was conflicted. It did not push forward with the apps partly because it saw the availability of Office for Windows devices, including Microsoft's own Surface family of tablets, as a way to differentiate its operating system.

It turned out that Office did not help much with sales of Surface, which first came out in late 2012. That may partly have been because Microsoft did not do enough to make Office work well with touch screens.

To Brad Silverberg, a venture capitalist in Seattle with Fuel Capital and a former senior Microsoft executive, the company's hesitation around bringing Office to the iPad is reminiscent of the early 1990s, when the makers of WordPerfect and Lotus 1-2-3, successful word-processing and spreadsheet applications, balked at making Windows versions of their products.

At the time, Silverberg said, he begged the companies to move faster to bring the products to Windows, but they didn't want to see Microsoft's software succeed. Microsoft exploited their hesitation.

"It gave Microsoft a huge opening to establish the Office franchise," Silverberg said. "Microsoft is in danger of doing the same kind of thing now."